1. Field of the Invention
The present invention relates to a glass ceramic which can provide a board comprising a metal substrate and the glass ceramic coated on said substrate, particularly a board comprising said board and a fine wiring pattern applied onto said board by means of thick film printing.
2. Discussion on Related Art
Thick film hybrid ICs and printed circuit board, have hitherto been prepared by using alumina boards or epoxy resin-glass boards. The Alumina boards have a defect of poor mechanical strength and of difficulty in producing large-sized boards. Epoxy resin-glass boards, on the other hand, are inexpensive and suited for large scale production but are unsatisfactory in thermal resistance, so that they have disadvantages in that the materials which can be used for circuit formation in said boards are limited to those for low temperature use (whereas the majority of thick film pastes require a firing temperature of 800.degree. to 900.degree. C.) and that the resulting products can be used only in an atmosphere of lower than 400.degree. C.
In order to obviate the disadvantages of the above-mentioned boards of the prior art, so-called porcelain enamel insulated boards formed by coating a glassy material on metal substrates have been proposed. One of the proposals relates to a board in which the porcelain enamel insulating layer comprises an amorphous glass containing a relatively small amount of alkali metal oxides (Na.sub.2 O, K.sub.2 O and Li.sub.2 O). The insulated boards of this type, however, have problems in re-softening, electric insulation and reliability of the insulating layer and in the accuracy of the circuit. In more particular, said boards have the following disadvantages: (1) when the porcelain enamel insulating layer is repeatedly fired, the insulating enamel layer will re-soften; (2) they are poor in electric insulation at high temperature as compared with prior boards (namely alumina boards); (3) when voltage is applied to the board at high temperature for long time, the circuit network is adversely affected owing to the migration of alkali ions; (4) the printed circuit undergoes contraction during the firing of the paste owing to the cohesive force of the paste in firing.
Another proposal relates to an attempt to use a crystallized glass for the porcelain enamel insulating layer. For example, as disclosed in U.S. Pat. No. 4,256,796, a porcelain enamel insulated board formed by coating on a steel plate a crystallized glass having the composition of 6-25% by mole of BaO, 30-60% by mole of a metal oxide (selected from the group consisting of MgO, ZnO and CaO, or a mixture of two or more thereof), 13-35% by mole of B.sub.2 O.sub.3 and 10-25% by mole of SiO.sub.2 undergoes no re-softening of the porcelain enamel layer even when printed with paste at 800.degree. to 900.degree. C. and subjected to repeated firing and moreover, since the glass is alkalifree, is excellent in electric insulation and reliability. Thus, the boards of the second proposal wholly excel those of the first proposal.
Though the composition is expressed in terms of % by mole in said U.S. Pat. No. 4,256,796, it roughly becomes as follows when calculated in terms of % by weight with reference to working examples disclosed in Japanese Patent Application corresponding to said U.S. Patent [namely, Japanese Patent Application Kokai (Laid-Open) No. 56-73643].
______________________________________ BaO 16-50% by weight MgO 16-42% by weight CaO 0-11% by weight ZnO 0-11% by weight CaO + ZnO 0-11% by weight B.sub.2 O.sub.3 12-34% by weight SiO.sub.2 10-23% by weight ZrO.sub.2 0-5% by weight Al.sub.2 O.sub.3 0-5% by weight SnO.sub.2 0-5% by weight ZrO.sub.2 + Al.sub.2 O.sub.3 + SnO.sub.2 0-5% by weight ______________________________________
However, the boards according to the second proposal have the following defects: (1) a poor thermal resistance; (2) a poor chemical resistance; (3) a large surface roughness, which makes the board unsuitable as a board for fine pattern wiring.
With regard to the defects (1) and (2) mentioned above, attempts have been made to ameliorate the defects by varying the composition of the glass, as disclosed in Japanese Patent Application Kokai (Laid-Open) Nos. 58-104042 and 60-172102, but they have not yet been put to practical use. This is because the boards obtained in above attempts have an extremely increased surface roughness as compared with those of the second proposals and hence cannot be used as a circuit board for thick film printing. Glass ceramics of MgO--BaO--SiO.sub.2 --B.sub.2 O.sub.3 type tend to undergo precipitation of the crystal phases of BaO.2MgO.2SiO.sub.2 and 2MgO.B.sub.2 O.sub.3 when fired at 800.degree. to 900.degree. C. and, in certain range of compositions, the ceramics undergo an extreme growth of crystal grains or precipitation of unusually large amount of crystals. These phenomena exert a great influence on the surface roughness of porcelain enamel insulated boards.
Thus, the most serious defect of the prior are glass ceramics is that, as shown in FIG. 2, the crystal grains increase their size during firing and, as a consequence, the surface roughness increases extremely.
In recent years, with the trend towards ICs of higher density and the advance in packaging technology, the requirements in circuit formation on boards have been increasingly directed toward higher density, higher integration and finer pattern.
In thermal heads, for example, a high density and a fine pattern of an electrode width of 30 .mu.m or less and an electrode interval of 200 .mu.m or less are required. Such requirements cannot be fulfilled by previous glass ceramics because of their too high surface roughness and have been coped with by lap-polishing the board. Such a means has been unable to satisfy the need for cost reduction, which is one of the advantageous features of porcelain enamel boards.